LONDON – A Northern Irish regional party that props up the United Kingdom’s minority Conservative government on Wednesday said the legal consequences of an Irish border backstop agreement proposed by the prime minister were unacceptable.

In a fresh blow to PM Theresa May, the right-wing Democratic Unionist Party said the border backstop, which would keep Northern Ireland in the customs union to avoid a hard border with the Republic of Ireland should talks collapse, would create an internal border within the UK.

“Northern Ireland will remain in the single market for goods and the EU’s customs regime and will be required to apply and comply with the relevant rules and standards. Herein opens up regulatory divergence in the future,” the pro-unionist party said.

The statement came after May’s government was forced by a parliamentary motion to publish a complete document on the legal consequences of Brexit.

“The implication, as outlined by the attorney general, of NI remaining in the EU single market for goods while GB (Great Britain) does not is that for regulatory purposes GB is ‘essentially treated as a third country by NI (Northern Ireland) for goods passing from GB into NI,’” the statement said.

“This is totally unacceptable and economically mad in that it will be erecting internal economic and trade barriers within the United Kingdom,” it continued.

The opposition to the government’s Brexit strategy could prove to be the latest in a string of headaches for the PM.

She relies on the DUP to pass government legislation through the Commons, the lower chamber of parliament in the UK.

Currently, the Tory leader was on a charm offensive as five-days of Brexit debates kicked off ahead of a key vote on the final deal on Dec. 11.

The UK is to leave the EU on March 29, 2019.

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